1. Technical Field
The invention relates to production of fused silica optical elements for use in transmitting ultraviolet radiation and use of the optical elements in microlithography systems.
2. Background Art
Excimer lasers and other lasers that operate in the short ultraviolet (UV) region are finding more application in microelectronics fabrication. In particular, excimer lasers are used in microlithography systems to produce integrated circuits with critical features in the sub-micron range. Generally, microlithography systems include a deep UV laser light source, an illumination lens system, and a projection lens system. The illumination system expands and homogenizes the intensity of the laser beam, and the imaging lens system projects a very high resolution image of a mask onto a photosensitized silicon wafer. Current microlithography systems use KrF excimer lasers, which operate at 248 nm, to produce integrated circuits with critical features of about 0.35 xcexcm and smaller. Shorter-wavelength lasers such as ArF excimer lasers can produce integrated circuits with even smaller critical features than possible with the KrF excimer lasers. However, the successful use of ArF lasers and other short UV radiation in microlithography systems depends on the availability of optical elements that can effectively transmit these wavelengths.
Fused silica glass has received a lot of attention lately for its ability to transmit short UV radiation. Unfortunately, the response of fused silica to prolonged exposure to high energy radiation is gradual monotonic development of absorption at the wavelength of the exposure beam. Electromagnetic radiation with wavelength as long as 248 nm is sufficiently energetic to induce absorption in the fused silica when applied to the fused silica at high doses. An exposure beam with wavelength as short as 193 nm is considerably more effective in producing induced absorption in the fused silica. Araujo et al. have demonstrated that this induced absorption is the result of photolysis of pre-existing defects in the glass structure, see, Araujo et al., xe2x80x9cInduced Absorption in Silicaxe2x80x94A preliminary Model,xe2x80x9d Proceedings of SPIExe2x80x94Inorganic Optical Materials, vol. 3424, 1998, pages 1-7. The photolysis leads to the creation of an Exe2x80x2 center, which is a single paramagnetic electron in a silicon orbital projecting into interstitial space of the glass structure. This Exe2x80x2 center has an absorption band that is roughly centered at a wavelength of 214 nm but extends to shorter wavelengths.
Induced absorption is particularly problematic for applications employing ArF lasers and other short UV radiation with wavelengths within the absorption band of the Exe2x80x2 center. The problem is two-fold. The first part of the problem is that the induced absorption attenuates the exposure beam, producing optical distortion. The second part of the problem stems from the fact that the absorption is constantly increasing over the duration of exposure. If optical distortion were the only problem, it would be possible to anticipate the induced absorption and compensate for it. However, because the induced absorption is constantly changing, this is not possible. Clearly, if it were possible to provide a way in which the induced absorption can quickly achieve a constant value, then it would be possible to compensate for phase changes in the exposure beam that is attributable to the induced absorption.
The invention is an optical member which comprises fused silica glass having a concentration of xe2x89xa1SiH moiety below detection limit as measured by Raman spectroscopy and a concentration of molecular hydrogen of at least 1xc3x971017 molecules/cm3. The fused silica glass exhibits an induced absorption level which quickly attains an initial peak upon exposure to irradiation and rapidly decays to a low value. The induced absorption level after decaying to the low value remains substantially unchanged by further irradiation.